Styrene is usually produced by dehydrogenation of ethylbenzene, which is used as a raw material monomer for synthetic rubber, ABS resin, polystyrene and the like, so that its production amount is increasing year by year.
The ethylbenzene dehydrogenation reaction is an endothermic reaction accompanied by expansion of volume as shown by the following reaction formula: EQU C.sub.6 H.sub.5.C.sub.2 H.sub.5 .fwdarw.C.sub.6 H.sub.5.C.sub.2 H.sub.3 +H.sub.2 - 113 kilojoules/mol
This reaction was actively studied in 1940th in the United States in order to meet the social requirement for the production of synthetic rubber, during which there has been technically established a system in which ethylbenzene is catalytically dehydrogenated under steam dilution as industrially carried out at present, resulting in occupying a position as a representative production method of styrene.
The volume is expanded in this reaction, so that it is advantageous from a viewpoint of chemical equilibrium to dilute the reactants with steam, and the steam dilution has the following advantages other than the above.
(a) The reaction is performed at a high temperature of 550.degree. C. to 650.degree. C., so that steam can be utilized as a heat source for heating ethylbenzene. PA1 (b) Although carbonaceous substances deposit on the catalyst due to side reactions, a water gas reaction with steam can be utilized for removing them, whereby continuous use can be conducted without regeneration of the catalyst. PA1 (c) The steam as the diluent can be easily separated from the product only by liquefying the product.
As described above, the dehydrogenation reaction system in the presence of steam is an industrially excellent production method in which styrene can be continuously produced under an advantageous condition from a viewpoint of chemical equilibrium, and the foregoing operation method has become technically available owing to the fact that it has been revealed that the iron oxide-potassium oxide system catalyst stably maintains its high performance without poisoning by steam, however, before the catalyst became industrially available, many further improvements in performance had been contemplated, during which addition of various promoter components had been investigated.
The role of each catalyst component has been scholarly elucidated under a situation of reaction, wherein the component which has an activity on the dehydrogenation reaction itself is partially reduced iron oxide, and potassium oxide acts as the promoter to enhance the activity of iron oxide and promotes the water gas reaction of steam with the carbonaceous substances deposited on the surface of the catalyst so as to prevent time-dependent deterioration of the activity, and other promoter components are added in order that the activity and selectivity are increased or the thermal stability, mechanical strength stability and the like of the catalyst are increased.
The catalyst is usually produced such that iron oxide or an iron compound as its precursor, a potassium compound and other promoter component oxides or precursor compounds thereof are mixed and kneaded in the co-presence of moisture, and then extrusion molding, drying and calcination are performed.
Those used as raw iron materials are red iron oxide (hematite) or yellow oxy-iron hydroxide (goethite) as its precursor compound and the like, and the raw potassium materials are those which can be decomposed into potassium oxide by calcination, for which any compound can be used provided that no component which gives a poisoning action is allowed to remain in the catalyst, however, potassium hydroxide, potassium carbonate or the like is usually used.
Iron oxide and potassium oxide are essential components provided that the ethylbenzene dehydrogenation reaction is performed in the presence of dilution steam, and the combination of the both components greatly enhances the activity of iron oxide as compared with the case in which it is used alone, however, only the both components were insufficient to use as an industrial catalyst, and in order to improve the activity as well as the selectivity, stability of catalyst structure, mechanical strength and the like, various promoter components to meet with the object have been added and supplied as commercial catalysts.
As the promoter components to be added, for example, are known Ce, Cr and the like as a component for increasing the activity, Ca, V, Mo, W and the like as a component for increasing the selectivity, and as the prior art in which these elements are used are proposed addition of Ce, Mo, Ca, Mg or Cr in U.S. Pat. No. 5,023,225, addition of Cr, Mo, W, V and Al in German Patent DE4025931, and addition of Ca, Ce, Ge, Sn, Pb and the like in Japanese Patent Laid-open No. 64-27646 respectively, while as components for contributing to the structural stability of the catalyst are known Cr, Mg and the like which are disclosed in U.S. Pat. No. 5,023,225 or DE4025931 together with the components for increasing the performance, however, as a component for stabilizing the catalyst structure being different from these elements, addition of Ti is disclosed in Czechoslovakia Patent CS168220 and 174488 respectively.
The addition of these promoter components greatly increases the catalyst performance and improves the stability of catalyst structure or mechanical strength, however, the dehydrogenation catalyst has a high alkali metal content and is used at a high reaction temperature in spite of the high alkali content, so that problems such as migration of the alkali metal in the catalyst, scattering toward the downstream side of the catalyst layer and the like are apt to take place in the practical operation, which result in the decrease in catalyst performance or the increase in pressure drop due to blockade of the catalyst layer, and hence a danger of giving a trouble for the operation of equipment is included, while its activity is fairly low as compared with the equilibrium conversion ratio of ethylbenzene at a practical industrial reaction temperature, remaining a room to be improved from a viewpoint of performance.
Here, when the ethylbenzene dehydrogenation reaction is considered from an industrial viewpoint, if the activity can be increased without deteriorating the selectivity of the catalyst, then not only the yield of styrene can be increased, but also an operation under a more moderate condition is made possible, so that it becomes possible to provide countermeasures for reducing various operational problems concerning the catalyst such as the decrease in activity due to sintering of iron oxide on account of thermal influences or migration of alkali metal, the increase in pressure drop due to scattering of alkali metal and the like.